Artificial disc prosthesis

ABSTRACT

A visco-elastic motion-limiting artificial intervertebral disc prosthesis is provided that mimics the physiologic function of a normal spinal disc. The disc comprises upper and lower endplates having therewithin channels or openings for optionally receiving one or more motion-limiting members fitted on each end with an enlarged portion. One or more compression stops is provided between the upper and lower endplates. Additionally, an elastomeric cushion is disposed between the endplates and surrounds the motion-limiting members. Also, force transducers and microelectronics can be utilized to provide data to the surgeon or the patient regarding the load state of the disc.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an artificial visco-elastic, constrainedmotion disc for replacing intervertebral discs in the lower back,particularly in the lumbar and lumbar-sacral regions.

2. Background

The human spine is composed of many vertebral bones stacked one upon theother, with an intervertebral disc between each pair of adjacentvertebral bones. The discs act as cartilaginous cushions and shockabsorbers. The spinal cord runs in a bony canal formed by successiveopenings in these bones. The spinal nerves exit the spinal cord betweenpairs of vertebrae and supply nerves and nerve signals to and from otherbody structures.

The intervertebral disc is a complex joint both anatomically andfunctionally. It is composed of three component structures: the nucleuspulposus; the annulus fibrosus, and the vertebral endplates. Thebiomedical composition and anatomical arrangements within thesecomponent structures are related to the biomechanical function of thedisc.

The nucleus pulposus, occupying about 25% to 40% of the total disccross-sectional area, usually contains approximately 70% to 90% water byweight. Because of this high water content, the nucleus may bemechanically described as an incompressible hydrostatic material.

The annulus fibrosus is a concentrically laminated structure whichcontains highly aligned collagen fibers and fibrocartilage embedded inan amorphous ground substance. The annular layers are oriented atapproximately +/−60° to the longitudinal axis of the spine. The annulusfibrosus usually contains approximately 8 to 12 layers, and ismechanically the main stabilizing structure which resists torsional andbending forces applied to the disc.

The two vertebral endplates separate the disc from the adjacentvertebral bodies, and are composed of hyaline cartilage.

Spinal discs may be damaged or displaced due to trauma or disease. Ineither case, the nucleus pulposus may herniate and protrude into thevertebral canal or intervertebral foramen. This condition is known as aherniated or “slipped” disc. The disc may in turn press upon the spinalnerve that exits the vertebral canal through the partially obstructedforamen, causing pain or paralysis in the area of its distribution. Themost frequent site of occurrence of a herniated disc is in the lowerlumbar region. To alleviate this condition, two procedures are common.

First, it may be necessary to remove the involved disc surgically andfuse the two adjacent vertebrae together. Spinal fusion is a good methodof eliminating symptoms, but at the expense of total loss of motion ofthe fused vertebral joint, as well as increased stress in the adjacentsegments. In many long-term patients of fused-spinal segments, adetrimental phenomenon has been observed whereby discs adjacent to thefused-spinal segment will have increased motion and stress due to theincreased stiffness of the fused segment. This is sometimes referred toas “cascading spine syndrome,” where previously normal motion segmentsabove or below a fused segment exhibit spondylolisthesis, ordegenerative disc disease due to increased loading.

A second method for alleviating disc problems is insertion of anintervertebral disc replacement. The object of an intervertebral discreplacement is to provide a prosthetic disc that combines both stabilityto support the high loads of the patient's vertebrae and flexibility toprovide the patient with sufficient mobility and proper spinal columnload distribution. In attempting to satisfy these competing designrequirements, basically four types of artificial intervertebral discshave been developed: elastomer discs, ball and socket discs, mechanicalspring discs, and hybrid discs.

Elastomer discs typically include an elastomer cushion which issandwiched between upper and lower rigid endplates. Elastomer discs canprovide cushion or damping functions similar in mechanical behavior tothe removed intervertebral disc tissue. However, known elastomer discsexperience long-term in-vivo problems stemming from micro-cracking,fixation problems with respect to the endplates, insufficientcompression and torsional resistance, and excessive motion which canlead to bulging of the replacement disc and resultant pain for thepatient. One hypothesis for the failures of previous elastomer baseddisc designs is the unlimited potential for strain. High load in vivoevents cause subsequent changes in structural characteristics of theelastomer, a characteristic called the Mullins effect. After initialstress softening effects are accounted for, a stable stress strain curveis reached. However, if a new load cycle is encountered exceeding theprevious peak strain, the structural properties will again change. Thisis the rationale for the failure of previous elastomer disc designs andthe inspiration for new motion-limited designs.

Ball and socket discs typically incorporate two plate members havingcooperating inner ball and socket portions allowing an articulatingmotion of the members during movement of the spine. These types of discsgenerally restore spinal motion, but inadequately replicate the naturalstiffness of the intervertebral disc. Furthermore, dislocation and wearproblems exist with these devices as well as unsatisfactory motionlimiting components. Some types also comprise polymers in conjunctionwith metallic components.

For example, a Link Charite disc includes polyethylene/cobalt chromemolybdenum (CCM) construction. This design restores motion, but in avery unphysiologic manner. The design is essentially a ball and socketjoint which does not provide the nonlinear elastic response of thenormal disc including hysteresis and therefore shock absorption. As inhip replacements this design is subject to wear and polyethylene debriscomplications. This disc, which has been extensively implanted in Europeand the United States, relies on a relatively incompressible ultra highmolecular weight polyethylene center mating with concave surfaces incobalt chrome upper and lower endplates. The mating surfaces provide alow friction pseudo ball-socket joint with motion constraints incompression and anterior-posterior as well as lateral translation. Thedevice is totally unconstrained in rotation about its axis, and intension. Though this device has a semi successful in vivo history, itlacks fundamental stress strain characteristics of the normal disc.Instead, it provides low friction, high movement, non-energy absorbingkinematic function restoration to the spine motion segment. The disc isdesigned to move freely until limits of travel are reached. The stoppingaction provided is very abrupt and thus loads the vertebral endplate ina shock-like manner at end-of-travel. This disc imitates a free movingball and socket joint, not a natural disc that behaves very elasticallyuntil annulus fibers play the role of a “limiter”. A natural disc isload sharing between the elastic elements and fibrous tissue limiters.

Mechanical spring discs, which generally have only two or three degreesof freedom, typically incorporate one or more coiled springs disposedbetween metal endplates. These discs generally allow movement of thevertebrae during flexion and extension. However, these types havesignificant wear problems, as well as problems dealing with in-vivotorsional loads, and overall these discs cannot replicate the six-degreeof freedom movement of a natural intervertebral disc.

Hybrid types of discs generally incorporate two or more principals ofany of the aforementioned disc types. For example, one common hybriddisc arrangement includes a ball and socket set surrounded by anon-adhered elastomer ring. This hybrid disc is more complex than wouldbe preferred for common usage, and more importantly, the intermittentcontact between the ball socket and the elastomer ring that occasionallyoccurs in-vivo causes critical wear problems.

It is to be recognized that an artificial disc constructed with apolymer between two metal endplates undergoes compression as a result ofboth gravity and patient activities requiring exertion of energy.Therefore, ideally a disc would include a means of protecting thepolymer and the possible bond joint between polymer and metal.Mechanical stops and motion-limiters can be added to maintain theintegrity of the prosthesis. Such structures can take the form of rods,tension cables, or other connectors, as well as metal-to-metal contactin compression, to name but a few examples. Moreover, it would bebeneficial for a disc also to include a means to convey to surgeons andto patients the actual state of the loads experienced by the device.

As a result, the need exists for an artificial intervertebral disc thatmore closely imitates a natural disc. This means that the artificialdisc should maintain the vertebrae spaced from each other and preventpinching of nerves or spinal cord. The artificial disc should providegood load distribution. Furthermore, the artificial disc should besufficiently resilient to accommodate other motions of the spine,including flexion, extension, lateral bending, and rotation, as well ascombinations of these motions. In humans, the bony facet joints actuallylimit the rotational movement. A disc typically need only rotateapproximately three degrees. Moreover, the disc should providerestorative force to bias toward the resting position. The artificialdisc should be both biocompatible and biostable such that the discitself or any of its degradation byproducts, if any, do not causeadverse tissue reactions. Ideally, through the use of strain gauges orother means of force transduction, the disc can also provide stored orreal-time data to the surgeon and the patient regarding the state of theloads and displacements experienced by the disc.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a prosthetic intervertebral discreplacement that restores and preserves the physiologic function of anormal spinal motion segment. The intervertebral disc replacementaddresses the detrimental phenomenon of cascading spine syndromeobserved in long-term patients with fused-spinal segments. The presentintervertebral disc replacement is designed to avoid the need for spinalfusion or at least prolong the need for it. The invention incorporateselements to bear high load in the design, while preserving the abilityof the elastomer to provide substantial motion at low to moderate loads.The invention demonstrates the potential to survive high cycle fatiguein bending, compression, and shear along all three mutually orthogonalaxes. The motion-limiting features survive high load, low cycle fatigueand preserve the integrity of the elastic range performance of thedevice.

According to the present invention, once the disc reaches itsmechanically constrained limits in compression, bending, and shear, thenthe elastomer and the bond interface experiences no additionalsignificant loads as the limiter feature will prevent furthersignificant strain. In vivo loading varies by the individual, activitylevel, and unique high load events. In the present invention, limitingstrains prevents the elastomer from continually changing its structuralcharacteristics with each high load event. A stable stress straincharacteristic can be reached and predictable disc performance can beachieved.

The present invention is a visco-elastic constrained-motion discprosthesis generally comprising specially designed rigid upper and lowerendplates having therewithin channels or openings for receiving one ormore motion-limiting members fitted on each end with an enlargedportion. In some embodiments, the lower surface of the upper endplatecontains a first projection therefrom that extends toward the lowerendplate. Likewise, the upper surface of the lower endplate can containa second projection extending toward said upper endplate andsubstantially aligned with said first projection. The first and secondprojections terminate to create a gap therebetween, forming acompression stop. Interposed between the upper and lower endplates is anelastomer cushion. Preferably, the elastomer cushion is not in directcontact with either the motion-limiting members or the first or secondprojections to avoid wear and debris problems.

The gap allows a predetermined amount of axial movement, but no more,between the upper endplate and the lower endplate. As a result, the gapprevents excess compression from occurring, and is usually designed toallow approximately 1 to 2 millimeters of relative movement between theupper and lower endplates. This limits the compressive stresses seen bythe elastomer. During most activities of the patient, the elastomer willexclusively carry the load. The compression stop will be engagedtypically only during activities of high exertion.

The motion-limiting members with enlarged portions are inserted intointernal cavities in the elastomer and link the upper endplate to thelower endplate. The motion-limiting members are dimensioned so as tohave a length that is slightly greater than the distance between thelower surface of the upper endplate and the upper surface of the lowerendplate (and also preferably slightly less than the overall length ofthe internal cavities in which they reside). This allows space for themotion-limiting members to move during compression.

In bending, which is the most important movement of an L4-L5 or L5-S1disc, the motion-limiting members are strategically oriented to resistthe tension in the posterior region of the disc. If motion-limitingmembers are present at the anterior portion of the disc, they floatfreely in the internal cavities during bending.

The external surfaces of the upper and lower endplates can also befitted with wedges, spikes, keels, or other appurtenances to aid inattachment to a vertebral body. These appurtenances can also serve ascovers to enclose the enlarged portions of the motion-limiting members.

Additionally, some embodiments of the invention utilize strain gauges,pressure transducers, piezoelectric force transducers, or other means offorce transduction to provide stored or real-time data to the surgeon orpatient of the load state of the disc.

Several commercially available low durometer (i.e., approximately 70-85A) polyurethanes with a history of animal and human implantation arecandidates to be used in a titanium or CoCrMo/elastomer construction.Hybrid discs according to the invention can overcome one of the failuremodes of previous artificial disc designs, namely that of delaminationand fatigue failure of the bonded interface between the elastomer andmetal. This can be accomplished through improved bonding andmotion-limiting features, in those embodiments where the visco-elasticcushion is bonded to the endplates. Other embodiments that utilize nobonding between the visco-elastic cushion and the endplates also achieveimproved results with motion-limiting features.

These and other benefits are obtained in the many embodiments of theinvention. A particularly useful embodiment comprises an artificialintervertebral disc prosthesis having an anterior portion and aposterior portion, further comprising: a first endplate having an uppersurface and a lower surface, wherein the first endplate furthercomprises at least one opening for receiving at least onemotion-limiting member; a first projection extending from the lowersurface of the first endplate terminating in a first distal end; asecond endplate having an upper surface and a lower surface, wherein thesecond endplate further comprises at least one opening for receiving atleast one motion-limiting member; a second projection extending from theupper surface of the second endplate and substantially aligned with thefirst projection, wherein the second projection terminates at a seconddistal end to form a gap having a predetermined distance between thefirst and second distal ends; at least one motion-limiting memberreceived respectively in the at least one opening of the first andsecond endplates, linking the two endplates and allowing only apredetermined amount of movement thereof; and a visco-elastic cushionbetween the first endplate and the second endplate, further comprisingtherein at least one cavity in substantial alignment with the at leastone opening in the first endplate and the second endplate through whichthe motion-limiting member may pass and at least one cavity surroundingthe first and second projections.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a graph showing the typical nonlinear response to load ofhuman spine motion segments;

FIG. 2 is a diagram showing a reference coordinate system for afunctional spinal unit to be used herein;

FIG. 3 is a posterior perspective view of a first embodiment of anartificial disc of the present invention.

FIG. 3(a) is the artificial disc of FIG. 3 showing a second type ofappurtenance fitted to the upper and lower endplates;

FIG. 3(b) is the artificial disc of FIG. 3 showing a third type ofappurtenance fitted to the upper and lower endplates;

FIG. 3(c) is the artificial disc of FIG. 3 showing a fourth type ofappurtenance fitted to the upper and lower endplates;

FIG. 4 is a top plan view of the artificial disc of FIG. 3;

FIG. 5 is a bottom plan view of the artificial disc of FIG. 3;

FIG. 6 is a posterior elevation view of the artificial disc of FIG. 3;

FIG. 7 is a lateral elevation view of the artificial disc of FIG. 3;

FIG. 8 is an exploded perspective view of the artificial disc shown inFIG. 3;

FIG. 9 is a section view taken along plane A-A in FIG. 3;

FIG. 10 is a section view taken along plane B-B in FIG. 3;

FIG. 11 is a section view taken along plane C-C in FIG. 3;

FIG. 12 is a section view taken along plane D-D in FIG. 3;

FIG. 13 is an exploded perspective view of an exemplary motion-limitingmember of a first embodiment of the invention;

FIG. 14 is a perspective view of a second embodiment of an artificialdisc of the present invention having four motion-limiting memberstherein;

FIG. 15 is an exploded perspective view of the embodiment of theartificial disc of the present invention shown in FIG. 14;

FIG. 16 is a section view taken along plane E-E in FIG. 14;

FIG. 17 is a section view taken along plane F-F in FIG. 14;

FIG. 18 is a section view taken along plane G-G in FIG. 14;

FIG. 19 is a section view taken along plane H-H in FIG. 14;

FIG. 20 is a section view taken along plane I-I in FIG. 14;

FIG. 21 is a side elevation view of a disc according to a firstembodiment of the invention showing the disc in normal bending mode;

FIG. 22 is a top view of an embodiment of a lower endplate flex circuitfor an artificial disc using strain gauges to provide force transductionfor providing data external to the disc;

FIG. 23 is a side section view of the disc shown in FIG. 21 in normalbending mode;

FIG. 24 is a perspective view of a motion-limiting member according toan embodiment of the invention incorporating a split ring in placearound the enlarged portion of the motion-limiting member;

FIG. 25 is a side elevation schematic of an embodiment of an artificialdisc using strain gauges to provide force transduction for providingdata to locations external to the disc;

FIG. 26 is a perspective view of the posterior side of an alternativeembodiment of the disc showing removable appurtenances;

FIG. 27 is a perspective view of the anterior side of the disc shown inFIG. 26;

FIG. 28 is a perspective view of the posterior side of an appurtenanceand an upper endplate according to an embodiment of the invention; and

FIG. 29 is a perspective view of the anterior side of the endplate shownin FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods are shown, it is to be understood from theoutset that persons of ordinary skill in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the description which follows is to beunderstood as illustrative and exemplary of specific embodiments withinthe broad scope of the present invention and not as limiting the scopeof the invention. In the following descriptions, like numbers refer tosimilar features or like elements throughout.

A successful intervertebral disc prosthesis should restore and preservephysiologic function of a normal spinal motion segment. FIG. 1 is agraph of the response of the normal human disc to load. The nonlinearresponse of the motion segment is a function not only of the disc, butof the facet joints and ligaments. Facet joint function and ligamentousstructures may be compromised and unable to provide load sharing as in anormal motion segment. The nonlinear response of spine motion segment toload shown in FIG. 1 is a typical curve shape in compression, shear,torsion, and bending.

FIG. 2 is a perspective view of a reference intervertebral disccoordinate system that will be used throughout this discussion for easeof reference. The figure shows typical loads (forces and moments) andtypical displacements (translation and rotation) that can occur in eachof the three mutually orthogonal directions. The load-deflection curveshape of FIG. 1 is similar in all three directions for a normal disc.Like that of a normal disc, a disc 10 of the present invention providesa nonlinear response to torsion, shear, and compressive loads.

Referring now to FIG. 3, the disc 10 comprises a first or upper endplate20, a second or lower endplate 30, and a visco-elastic cushion 40interposed between and adhered to the two endplates. The upper and lowerplates 20, 30 are substantially symmetrical about an anterior-posteriorhorizontally extending plane (a transverse plane shown in FIG. 2), aswell as about a sagittal plane (FIG. 2). The terms “upper” and “lower”are used herein only for illustration purposes with reference to theorientation of the disc 10 when it is implanted in the human bodybetween two adjacent vertebrae V1 and V2 (defined as the cephalad-caudaldirection in FIG. 2). Indeed, the upper plate is more generallydescribed as a first plate and the lower plate is more generallydescribed as a second plate.

The upper endplate 20 is rigid and is preferably made from abiocompatible material such as stainless steel, titanium, titaniumalloys (such as Ti6Al4V), composite materials, and the like. The mostpreferred material is cobalt chrome molybdenum (CoCrMo or “CCM”)comprising approximately 66% Co, 28% Cr, and 6% Mo by weight.

The upper endplate 20 has an upper surface 21 and a lower surface 22 andan anterior portion 23 and a posterior portion 24. Upper surface 21 andlower surface 22 are generally parallel. The anterior portion 23 is theportion of the upper endplate 20 that is disposed anteriorly in thespine when the disc 10 is implanted. Likewise, the posterior portion 24is the portion of the upper endplate 20 that is disposed posteriorly inthe spine when the disc 10 is implanted. The upper endplate 20 has anexternal surface 29 therearound that preferably defines a generally “D”shape. In one embodiment of the invention, the posterior portion 24 ofthe external surface 29 has a concavity 28 therein that definesposterior lobes 25, 26 projecting from the posterior portion 24 (seeFIGS. 3, 4, 5, 12, 14, and 15). One or more appurtenance 27 maybeoptionally affixed to the upper surface 21 to facilitate attachment.

The upper endplate 20 likewise may comprise an upper subplate 200 (seeFIG. 15) that can be formed monolithic with the upper endplate 20 or asa separate component affixed thereto subsequent to manufacture.Alternatively, the structures that comprise the upper subplate 200 maysimply be included in the upper endplate 20. The remainder of thisdescription will discuss structure related to subplate 200, but itshould be recognized that the description applies equally to discshaving no separate subplate. The subplate 200 further comprises an uppersurface 210 and a lower surface 220 and an anterior portion 230 and aposterior portion 240. A first projection 270 optionally may depend fromthe lower surface 220 of the subplate 200 to act as part of acompression stop, as will be described below. The upper subplate 200also has a plurality of openings 271 therethrough for receiving one ormore motion-limiting members 80 (described below). Preferably, the uppersubplate 200 includes two openings 271, one disposed posteriorly andslightly to the left (in the medial-lateral plane) of the firstprojection 270 and another disposed posteriorly and slightly to theright (in the medial-lateral plane) of the first projection 270(assuming the center of rotation is at the geometric center of thedisc). The openings 271 further comprise a bearing surface 272 forinteracting with the motion-limiting members 80 or a split ring assembly400 (described below). The bearing surface 272 is preferably a taperedopening having a larger diameter at the upper surface 210 than at thelower surface 220. The taper can be linear or nonlinear, including conicsections, parabolic sections, spherical sections, and so forth, to nameonly a few examples.

The first projection 270 preferably extends from said lower surface 220a height of approximately 1 mm to approximately 3 mm. Many shapes arepossible for the first projection 270, and indeed multiple projections,or no projections, are contemplated as well. In the preferredembodiment, the first projection 270 takes the form of a substantiallycylindrical section having a slight radius on its terminal end ofapproximately 2 mm to approximately 15 mm, preferably approximately 8 mmto approximately 12 mm.

In like manner, the disc 10 further comprises a lower endplate 30. Thelower endplate 30 is rigid and is preferably made from a biocompatiblematerial such as stainless steel, titanium, titanium alloys (such asTi6Al4V), composite materials, and the like. The preferred material iscobalt chrome molybdenum (CCM) comprising approximately 66% Co, 28% Cr,and 6% Mo by weight, respectively.

The lower endplate 30 has an upper surface 31 and a lower surface 32 andan anterior portion 33 and a posterior portion 34. Upper surface 31 andlower surface 32 are generally parallel. The anterior portion 33 is theportion of the lower endplate 30 that is disposed anteriorly in thespine when the disc 10 is implanted. Likewise, the posterior portion 34is the portion of the lower endplate 30 that is disposed posteriorly inthe spine when the disc 10 is implanted. The lower endplate 30 has anexternal surface 39 therearound that preferably defines a generally “D”shape. In one embodiment of the invention, the posterior portion 34 ofthe external surface 39 has a concavity 38 therein that definesposterior lobes 35, 36 projecting from the posterior portion 34 (seeFIGS. 3, 4, 5, 12, 14, and 15). One or more appurtenance 37 may beoptionally affixed to the lower surface 32 to facilitate attachment.

Although not preferred, it is possible that the lower endplate 30 maycomprise a lower subplate 300 (see FIGS. 8 and 15) that can be formedmonolithic with the lower endplate 30 or as a separate component affixedthereto subsequent to manufacture. The subplate 300 further comprises anupper surface 310 and a lower surface 320 and an anterior portion 330and a posterior portion 340. A second projection 370 depends from theupper surface 310 of the subplate 300 to act as part of a compressionstop, as will be described below. The lower subplate 300 also has aplurality of openings 371 therethrough, having bearing surfaces 372, forreceiving one or more motion-limiting members 80 (described below).Preferably, the lower subplate 300 includes two openings 371, onedisposed posteriorly and slightly to the left of the first projection370 and another disposed posteriorly and slightly to the right of thefirst projection 370.

The second projection 370 preferably extends from said upper surface 310a height of approximately 3 mm to approximately 6 mm. Preferably thesecond projection 370 is in substantial alignment with the firstprojection 270. Stated otherwise, the second projection 370 preferablywill have its longitudinal axis aligned with or close to thelongitudinal axis of the first projection 270. This is not mandatory,however. Indeed, the two projections 270, 370 may be offset from oneanother, it being more important that at least a portion of theprojections 270, 370 overlap during contact therebetween. And, dependingon the respective shapes of the projections 270, 370, the amount ofoffset may vary. Many shapes are possible for the second projection 370,including, but not limited to, all regular polygonal shapes.Additionally, the projections 270, 370 may take the form of partialpolygons (for example, a half cylinder or a partial elliptical cylinder,to name but a few). In the preferred embodiment, the second projection370 takes the form of a cylindrical platform having a diameter ofapproximately 6 mm to 10 mm, and more particularly, approximately 7 mmto approximately 9 mm.

Referring now to FIGS. 8, 9, 12 and 13, a motion-limiting member 80typically resides within each opening 271, 371. Each motion-limitingmember 80 has a length 81 and a diameter 82, and a first end 83 and asecond end 84. At each end 83, 84 is an enlarged portion 90. Themotion-limiting members 80 link the upper endplate 20 to the lowerendplate 30 and assist in handling the loads associated with flexion, aswill be described below.

The motion-limiting members 80 can be any of several longitudinalrod-like members, both rigid and semi-rigid, including solid metallicbars or rods of varying cross-sections, and wire. If wire is used as themotion-limiting member 80, the motion-limiting members 80 typically havediameters of approximately 0.038 inches to approximately 0.080 inches.However, the number of motion-limiting members 80 used plays a role indetermining the diameter of each motion-limiting member 80. In thepreferred embodiment, there are two motion-limiting members 80 that arebraided metal wires, preferably a braided stainless steel wire having adiameter of approximately 0.062 inches and a rated tensile strength ofapproximately 320 pounds. More specifically, the motion-limiting members80 can be of any material described above, but are preferably cables of316L stainless, MP35N, Haynes 25. In alternative, though less preferred,embodiments where substantially more numerous motion-limiting members 80are used (for example ten to twenty), the diameters can be significantlysmaller.

The enlarged portion 90 at each end 83, 84 is typically aspherically-shaped structure, or ball 91, that is affixed to themotion-limiting member 80. Balls 91 are preferably of the same materialas the motion-limiting members 80. The ball 91 has an upper surface 92and a lower surface 93. Preferably, ball 91 is preformed onto themotion-limiting member 80. However, methods of fixation are also variedand include welding, both during formation of the ball 91 at the firstend 83 and during assembly of the ball 91 at the second end 84; as wellas crimping on a ball 91. The balls 91 are preferably hemisphereswherein the lower surface 93 engages the bearing surface 272 of theplates 20, 30 and/or the subplate 200, 300. The upper surface 92 of theball 91 provides little to no advantage and merely takes up space. As aresult, the upper surface 92 is preferably flat or very low-profile soas to take up a minimal amount of space. The appurtenances 27, 37 on theupper and lower endplates 20, 30, respectively, may be used to cover aportion of the enlarged portions 91. In some embodiments, however, theinside surface 400 d of the split ring assembly (described below) can beused to effectively shorten the length of the motion-limiting member 80such that no part of the enlarged portion 91 extends beyond the uppersurface 21 of the upper endplate or the lower surface 32 of the lowerendplate 30.

In the preferred embodiment, a split ring assembly 400, as shown inFIGS. 8, 13, 15, and 24, fits into the openings 271 of the uppersubplate 200 and into openings 371 of the lower subplate 300. The splitring assembly 400 comprises a first half 400 a and a second half 400 b.When the two halves 400 a, 400 b of the split ring assembly 400 arecombined, they form a ring-shaped member having a port 400 c defined bya periphery 400 e. The split ring assembly 400 includes an insidesurface 400 d that serves as a bearing surface against which the balls91 can articulate, resulting in a mini-ball and socket joint. Thediameter of port 400 c is smaller than that of the ball 91. Thus, whenassembling the artificial disc prosthesis, the motion-limiting member 80is inserted into the opening 271 of the upper subplate 200 at the firstend 83 and into the opening 371 of the lower subplate 300 at the second84. The first half 400 a of the split ring assembly 400 is then insertedinto the opening 271 underneath the ball 91 at the first end 83 of themotion-limiting member 80. Then, the second half 400 b of the split ringassembly 400 is inserted into the opening 271 of the upper subplate 200underneath the ball 91 at the first end 83 of the motion-limiting member80, completing the split ring assembly 400 in the upper subplate 200.Since the diameter of the port 400 c in the split ring assembly issmaller than that of the ball 91 at the first end 83 of themotion-limiting member 80, the motion-limiting member 80 is preventedfrom slipping through the opening 271 of the upper subplate 270.

In the same manner, a split ring assembly 400 is inserted into theopening 371 of the lower subplate 300 above the ball 91 at the secondend 84 of the motion-limiting member 80 in order to prevent themotion-limiting member 80 from slipping through the opening 370 of thelower subplate 300. Once the split ring assemblies 400 are in place,they may be welded or permanently affixed by some other means known inthe art to the upper and lower subplate assemblies 200, 300. The splitring assembly 400 includes an inside surface 400 d that serves as abearing surface against which the balls 91 can articulate, resulting ina mini-ball and socket joint, thus minimizing the bending of themotion-limiting member 80 and extending fatigue life.

In another embodiment, the split ring assembly 400 is not present, andthe balls 91 are not preformed onto the motion-limiting members 80.Instead the balls 91 are fixed onto the motion-limiting members 80through some other means such as welding or crimping, as discussedabove. However, welding the ends of the motion-limiting members 80 intoballs 91 can lower the strength of the motion-limiting members 80 by 10to 15 percent. In this embodiment, the diameter of ball 91 can becontrolled during welding, ranging in size from approximately slightlylarger than the diameter of the motion-limiting member 80 up to amaximum diameter of approximately two times the motion-limiting memberdiameter. In embodiments using other attachment methods, other diametersare possible. Since no split ring assembly 400 is present in thisembodiment to prevent the motion-limiting members 80 from slipping outof the openings 271, 371 of the upper and lower subplates 200, 300,respectively, it is necessary that the diameter of the balls 91 belarger than the diameter of the opening 271 at the lower surface 220 ofthe upper subplate 200 and the opening 371 at the upper surface 310 ofthe lower subplate 300.

In the embodiments containing no split ring assembly, the balls 91 atthe first end 83 and the second end 84 mate with the bearing surface 272of the openings 271 in each endplate 20, 30 and/or subplate 200, 300. Asa result, each union of ball 91 and bearing surface 272 results in amini-ball and socket joint that allows articulation of the ball 91 andmotion-limiting member 80 within the opening 271, thus limiting bendingof the motion-limiting member 80, thereby extending fatigue life.

The disc 10 of the preferred embodiment provides motion-limitingfeatures in compression and bending, thereby behaving in vivo in afashion more similar to a natural spinal disc. The first projection 270and the second projection 370, by way of the predetermined gap 470therebetween (see FIGS. 9, 10, 11, 16, 17, and 19), provide acompression stop preventing movement between the upper and lower plates20, 30 in a magnitude greater than a predetermined amount. Varyinglimits of movement may be set, depending on the type and location of thedisc 10 in the body. In the preferred embodiment, the gap 470 betweenthe first and second projections 270, 370 is approximately 1 to 2millimeters. Elastomers that can handle excessive compressive loadingsmay be able to accommodate a gap greater than 2 millimeters. Once acompressive load is applied to the disc 10, the visco-elastic cushion 40absorbs the compression in a visco-elastic fashion according to designproperties of the elastomer. Upon absorbing the compressive loads inamounts sufficient to cause relative movement between the upper endplate20 and the lower endplate 30 in an amount equal to the gap 470, thefirst and second projections 270, 370 then come into contact and preventfurther compressive loads from being applied to the elastomer. It isexpected that the stop mechanism will only be activated during the moststrenuous of activities of the patient.

In bending, the motion-limiting members 80 can perform the primary orsecondary motion limiting functions. Referring to FIG. 2, the mostcommon bending scenario for the spine is bending in the sagittal plane(that is, bending about the x-axis). For example, this would beaccomplished by bending over to tie one's shoes. As a bending moment isapplied to the disc 10, the disc 10 can undergo both shear and lateraldisplacement. Referring now to FIGS. 21 and 23, a disc 10 is depicted insuch a scenario and is shown exaggerated for easier reference. It can beseen that the anterior side is slightly compressed, while the posteriorside is slightly in tension and a slight translation of the upperendplate 20 with respect the lower endplate 30. It can be seen that themotion-limiting members 80 can become oriented diagonally. In thisevent, the motion-limiting members 80 in tension provide a forcepreventing the upper endplate 20 and lower endplate 30 from separatingbecause the lower surface 93 of the ball 91 begins to bear upon thesplit ring assembly 400 and/or bearing surfaces 272 of the openings 271.It should be noted that the mechanics of the disc 10 depicted in FIGS.21 and 23, with reference to FIG. 2, are basically identical whether thebending is in the sagittal plane (front to back, or tying ones shoes) orin the frontal plane (side to side bending).

In flexion, which is the most important movement of an L4-L5 or L5-S1disc, the motion-limiting members 80 are strategically oriented toresist the tension in the posterior region of the disc 10. When braidedcables are used as the motion-limiting members 80, compression of thecables can cause splaying, which shortens their fatigue life. Therefore,placing more motion-limiting members 80 posteriorly than anteriorly (forexample, see FIG. 12) accounts for the greater expected flexion momentsof 20-30 N·m versus the lesser extension moments of about 10 N·m.

A further factor in the allowed range of motion in flexion and extensionis a consideration of the cable distance from an assumed center ofrotation at the center of the disc 10. The farther from the center, thegreater the resisting moment, but the more initial slack necessary toallow the required 8-12 degrees of flexion. The more initial slackallowed, the more bending movement is allowed. Combinations of cableplacement and initial diagonal orientation may be necessary to solvethese conflicting design goals. Although greater ranges are certainlypossible for the invention, preferable ranges of stiffnesses and motionfor the disc 10 are as follows: Nonlinear stiffness in compression(1,000 to 3,000 N/mm) and in flexion (1.0 to 5.0 N·m/deg) and maximummotion in compression (1.0 to 2.0 mm) and in flexion (8 to 12 deg).

In one embodiment of the artificial intervertebral disc prosthesis (seeFIGS. 14, 15, 16, 17, 18, and 19), four motion-limiting members arepresent. In this embodiment, there are corresponding four openings 271,371 in both the upper and lower subplates, 200, 300. One of the openings271, 371 is located anteriorly and slightly to the left of the first andsecond projections 270, 370, while another opening 271, 371 is locatedanteriorly and slightly to the right of the first and second projections270, 370. Similarly, a third opening 271, 371 is located posteriorly andslightly to the left of the first and second projections 270, 370, whilethe fourth opening 271, 371 is located posteriorly and slightly to theright of the first and second projection 270, 370. In this embodiment,when the disc 10 is in flexion, the most important movement of an L4-L5disc, the motion-limiting members 80 at the posterior portion of thedisc 10 are strategically-oriented to resist the tension in theposterior region of the disc 10, while the motion-limiting members 80 atthe anterior portion of the disc 10 float freely in the spike cavity inthe anterior region of the disc 10. However, a disadvantage of thisembodiment is the presence of motion-limiting members 80 at the anteriorportion of the disc 10. As discussed above, when braided cables are usedfor the motion-limiting members 80, compression can cause splaying,leading to a shorter fatigue life. As motion-limiting members 80 locatedat the anterior of the disc 10 encounter significantly more compressionthan motion-limiting members 80 located at the posterior of the disc 10during the normal activity patterns of most individuals,anteriorly-located motion-limiting members 80 are likely to have ashorter life than those located at the posterior of the disc 10.

Another embodiment of the artificial disc intervertebral prosthesis isenvisioned in which no motion-limiting member 80 is present. Instead,the compression stop formed by the first and second projections 270, 370also limits motion in flexion and lateral bending. This is accomplishedby sizing the first and second projections 270, 370 such that when theprosthesis engages in flexion or bending and the first projection 270tilts toward the second projection 370, the leading edge of the firstprojection 270 will come into contact with the second projection 370after a predetermined amount of flexion or bending, preventing furthermotion. This has the advantage of eliminating the need for themotion-limiting members 80, thereby reducing the number of elementswithin the prosthesis that are susceptible to wear and fatigue.

The preferred disc has certain load versus deflection characteristicsthat are similar to those found in the natural human disc. As was statedabove, it is useful, once implanted, that the surgeon and patient canknow the state of load experienced by the device. In this regard,embodiments of the disc have, integral to its construction, straingauges or other means of force or pressure transduction. Forillustration purposes only, not to be construed as limiting theinvention thereto, the discussion will be directed to the use of straingauges. A transducer can be connected to signal conditioning andamplification circuitry on a micro scale in order to fit within theconstraints of space available in the upper or lower endplate. In thisembodiment, the center stop is integral to the lower endplate andconsists of a hollow raised cylindrical platform. The space inside thiscylinder can house, for example, a 3×3×3 mm electronics package. Thepackage can be wired to strain gauges on the inside of the raisedcylinder and in peripheral locations around the bottom endplate.Alternatively, the transduction means can be connected to electronicssuch as piezoelectronics that eliminate the need for signal conditioningand amplification.

Since the center stop cylinder is not in contact with the polymer, thestrain gauge placed there will only measure contacts between the upperendplate and the center stop on the lower endplate. This data is usefulin itself as an indicator of when loads are sufficiently high to engagethe stop mechanism. However, in addition, more continuous data isavailable from peripherally placed strain gauges that will measurestress in the endplate caused by compression, bending, torsion, andshear loads in all directions. This information can give a precisemeasure of the magnitude and direction of loads on the disc. FIGS. 22and 25 depict how one embodiment of an artificial disc 10 uses straingauges to measure the load experienced by the prosthesis and relay thatdata on demand.

Placing the transducers within the polymer is not preferred sincefatigue of wires or the introduction of stress risers within the polymercan lead to fatigue failure initiation sites. For these reasons, placingthe signal conditioning microelectronics within the center compressionstop is advantageous. Once conditioned, the signal is telemeterized ondemand through excitation of an internal coil from an inductivelycoupled external coil. This couple energizes the electronics andtransmits the data upon request. This mode of interrogating the devicefor its load condition can be done real time or fed back from memorystorage according to a preset sampling routine. It is also possible tosample the device remotely and wirelessly via the internet.

The data can indicate changes in the device since its implantation. Itcan also store load history to indicate if the patient is followingdoctor's orders for allowed activities. The power source formemory-based data is optionally a micro battery or a capacitor chargedfrom the external inductive couple. The use of piezos is also possible.In one embodiment, a mylar flex circuit is pre-made and placed on thesecond endplate. Transducers are embedded on the mylar circuit andconnected to the signal conditioning and amplifying electronics.

FIGS. 26-29 show an alternative embodiment of the invention wherein thedisc 10 comprises multiple components that may be implanted separately.Indeed, the disc 10 may comprise many forms and embodiments that areimplantable in pieces. The desirability of multi-part prostheses isknown by surgeons and simplifies the implantation procedures. FIGS.26-29 simply depict one example. In these figures, the appurtenances 27are removably insertable into the upper and lower endplates 20, 30. InFIG. 26 the upper surface 92 of the balls 91 is not depicted buttypically would be visible. Preferably, the groove into whichappurtenances 27 are slid form a taper lock fit to lock the appurtenanceinto place. One example of a surgical procedure using a multi-part disc10 involves the surgeon first removing the diseased or damaged discusing the typical procedures in practice. An instrument (not shown) isused to prepare the site by cutting grooves into the adjacent vertebraeto receive the appurtenances 27 of the disc 10. Preferably theinstrument cuts both the upper and lower grooves simultaneously so as tomaintain the tolerances needed for the implantation of the disc 10. Aninstrument then inserts the appurtenances 27 into the prepared site. Thedisc 10 is then inserted between the appurtenances 27, which receive thegrooves of the upper and lower endplates 20, 30. The taper lock securesthe appurtenances 27 to the prosthesis.

Many possibilities exist for the manufacturing and the materialsinvolved in an artificial intervertebral disc prosthesis according tothe present invention. The endplates 20, 30 and/or endplate subplates200, 300 may be of the materials described above. Further, they may havethicknesses in the range of approximately 1 mm to approximately 3 mm.Their surfaces may be surface-treated or machined for texture andbonding improvement. Examples of such treatments include but are notlimited to ion etching, simple grit blasting, plasma spraying, or CNCmachined geometry. Preferably the endplates 20, 30 (and/or 200, 300) area CCM which is good in wear. The upper surfaces 21, 210 of the upperendplate 20 and upper subplate 200, and the lower surfaces 31, 310 ofthe lower endplate 30 and lower subplate 300, as well as the surfacesthat interact with the visco-elastic cushion, may be coated withTitanium 6Al4V to improve bone interface and bonding. The secondprojection 370 can, as stated above, take many forms, including by wayof mere examples, a cylinder, a post, a platform, and so forth. Thepreferred cylindrical member is a solid projection from the lowerendplate 20 or lower subplate 200. However, the cylindrical member couldbe hollow so as to accommodate integral microelectronics diagnostics, aswas discussed above.

Manufacturing the disc 10 can be accomplished in a variety of manners.Preferably, the endplates 20, 30 are first machined of either titaniumor cobalt chrome molybdenum (CCM), with openings 270 representing areaswhere there will be no elastomer cushion 40. The endplates 20, 30 areinserted into a mold with cores to create voids that will later containthe center stop and the motion-limiting members 80. It is preferable tokeep the elastomer free of the cables and center stop so as not tointroduce elastomer abrasion leading to fatigue failures. Themotion-limiting members 80 are then assembled to the molded subassemblywith a welding process. The cable assemblies terminate in a ball end. Inthe preferred embodiment the ball is preformed onto the cable, thoughthis is not required.

These ball-cable assemblies are then inserted into the endplates 20, 30with the split ring assemblies 400 as described above. In thoseembodiments not utilizing split ring assemblies 400, the ball-cableassemblies are inserted into the endplates 20, 30 as described above.Preferably, over each ball location on the upper endplate 20 and on thelower endplate 30 are appurtenances 27, 37, respectively. Theseappurtenances are shown in FIG. 3 as keels, but they could be individualconical spikes as shown in FIG. 3(a) or other appurtenances, for exampleFIGS. 3(b) and 3(c). For those embodiments where the enlarged portion 90extends beyond the plane of the upper surface 21 of the upper endplate20 or the lower surface 32 of the lower endplate 30, these appurtenancesserve as temporary anchors in the vertebra and covers that enclose themini ball-socket joint created between the endplate and the ball.Additionally, the ball-socket articulation prevents bending in thecables, thereby extending fatigue life. In certain embodiments utilizingmany motion-limiting members 80, it is possible for the motion-limitingmembers 80 to be laser welded into the endplates in the openings wherecores created voids in the elastomer.

As stated above, the center stop is a designed gap preferably to preventmore than 1-2 mm of compression from occurring, thereby limiting theelastomer compressive stress. This mandates a good wear interface forthe stop. A choice of CCM on CCM is preferred due to its recentintroduction as the wear couple in some FDA-approved metal-on-metalhips. Also as stated, many structural configurations for the first andsecond protrusions 270, 370 are possible, including pin-on-pin,pin-on-plate (shown), plate-on-plate, ball-on-plate, and so forth. Theelastomer will exclusively carry the load during most activities ofdaily living. The center stop will be engaged only during activities ofhigh exertion, except in the embodiment in which no motion-limitingmembers 80 are present.

While there has been described and illustrated particular embodiments ofa novel artificial disc prosthesis, and in particular, a visco-elasticconstrained motion disc, it will be apparent to those skilled in the artthat variations and modifications may be possible without deviating fromthe broad spirit and principle of the present invention, which shall belimited solely by the scope of the claims appended hereto.

1. An artificial intervertebral disc prosthesis having an anteriorportion and a posterior portion, comprising: a first endplate having anupper surface and a lower surface, said first endplate furthercomprising at least one opening; a first projection extending from saidlower surface of said first endplate terminating in a first distal end;a second endplate having an upper surface and a lower surface, saidsecond endplate further comprising at least one opening; a secondprojection extending from said upper surface of said second endplate andsubstantially aligned with said first projection, said second projectionterminating at a second distal end to form a gap having a predetermineddistance between said first distal end and said second distal end; atleast one motion-limiting member received respectively in said at leastone opening of said first endplate and said second endplate, said atleast one motion-limiting member linking said first endplate to saidsecond endplate and allowing only a predetermined amount of movement ofsaid first endplate relative to said second endplate; and avisco-elastic cushion interposed between said first endplate and saidsecond endplate, further comprising therein at least one cavity insubstantial alignment with said at least one opening in said firstendplate and said second endplate through which said motion-limitingmember may pass, and said at least one cavity surrounding said first andsecond projections.
 2. The artificial intervertebral disc prosthesis ofclaim 1 wherein said first endplate further comprises a first subplatehaving an upper surface and a lower surface and at least one openingtherethrough for respectively receiving said at least onemotion-limiting member, wherein said first projection of said firstendplate extends from said lower surface of said upper subplate.
 3. Theartificial intervertebral disc prosthesis of claim 1 wherein said secondendplate further comprises a second subplate having an upper surface anda lower surface, wherein said second projection of said second endplateextends from said upper surface of said second endplate.
 4. Theartificial intervertebral disc prosthesis of claim 1 wherein said atleast one opening of said first endplate has a first diameter near saidupper surface of said first endplate and a second diameter at said lowersurface of said first endplate, wherein said first diameter is greaterthan said second diameter.
 5. The artificial intervertebral discprosthesis of claim 1 wherein said at least one opening of said secondendplate has a third diameter at said lower surface of said secondendplate and a fourth diameter at said upper surface of said secondendplate, wherein said third diameter is greater than said fourthdiameter.
 6. The artificial intervertebral disc prosthesis of claim 1wherein said first projection of said first endplate extends a distanceof approximately 1 to approximately 3 millimeters from said lowersurface of said first endplate.
 7. The artificial intervertebral discprosthesis of claim 1 wherein said first projection of said firstendplate is substantially cylindrically shaped.
 8. The artificialintervertebral disc prosthesis of claim 1 wherein said first distal endhas a radius of approximately 2 millimeters to approximately 15millimeters.
 9. The artificial intervertebral disc prosthesis of claim 1wherein said second projection of said second endplate extends adistance of approximately 3 millimeters to approximately 6 millimetersfrom said upper surface of said second endplate to said second distalend.
 10. The artificial intervertebral disc prosthesis of claim 1wherein said gap between said first distal end and said second distalend at unloaded conditions is approximately 1 millimeters toapproximately 2 millimeters.
 11. The artificial intervertebral discprosthesis of claim 1 wherein said motion-limiting member is alongitudinal member having a first end and a second end furthercomprising a first enlarged portion at said first end and a secondenlarged portion at said second end.
 12. The artificial intervertebraldisc prosthesis of claim 11 wherein said at least one opening of saidfirst endplate has a first diameter near said upper surface of saidfirst endplate and a second diameter at said lower surface of said firstendplate, wherein said first diameter is greater than said seconddiameter, and wherein said at least one opening of said second endplatehas a third diameter at said lower surface of said second endplate and afourth diameter at said upper surface of said second endplate, whereinsaid third diameter is greater than said fourth diameter, and whereinsaid first enlarged portion has a diameter greater than said seconddiameter and wherein said second enlarged portion has a diameter greaterthan said fourth diameter.
 13. The artificial intervertebral discprosthesis of claim 1, further comprising a split ring assembly havingan upper surface and a lower surface and a through hole therein.
 14. Theartificial intervertebral disc prosthesis of claim 13 wherein said splitring assembly is fitted into said at least one opening at said uppersurface of said first endplate and said at least one opening at saidlower surface of said second endplate.
 15. The artificial intervertebraldisc prosthesis of claim 14 wherein said through hole at said lowersurface of said split ring assembly has a smaller diameter than saidfirst opening of said first endplate at its lower surface.
 16. Theartificial intervertebral disc prosthesis of claim 14, wherein saidthrough hole at said lower surface of said split ring assembly has asmaller diameter than said second opening of said second endplate at itsupper surface.
 17. The artificial intervertebral disc prosthesis ofclaim 14, wherein said motion-limiting member comprises a longitudinalmember having a first end and a second end and an enlarged portion atboth said first end and second end, said enlarged portions having adiameter larger than that of said opening of said lower surface of saidsplit ring assembly.
 18. The artificial intervertebral disc prosthesisof claim 1 wherein said upper surface of said first endplate and saidlower surface of said second endplate further comprise appurtenancesthat aid in securing the prosthesis to adjacent vertebrae.
 19. Theartificial intervertebral disc prosthesis of claim 1, further comprisinga force or pressure transducer located within said prosthesis forallowing the measurement and transmittal of information about loadsexperienced by the prosthesis.
 20. The artificial intervertebral discprosthesis of claim 19, wherein said second projection of said secondendplate houses at least a portion of a package of signal conditioningand amplification electronics that is connected to said force orpressure transducers placed within said second projection or at otherperipheral locations around said second endplate.
 21. The artificialintervertebral disc prosthesis of claim 19, wherein said secondprojection of said second endplate houses electronics that are connectedto said force or pressure transducers placed within said secondprojection or at other peripheral locations around said second endplate.22. The artificial intervertebral disc prosthesis of claim 19, whereinsaid second endplate further comprises a flex circuit including a loador pressure sensor embedded onto said upper surface of said secondendplate.
 23. The artificial intervertebral disc prosthesis of claim 1wherein said first endplate and said second endplate comprise abiocompatible material suitable for implantation.
 24. The artificialintervertebral disc prosthesis of claim 23 wherein said first endplateand said second endplate comprise materials selected from the groupconsisting of stainless steel, stainless steel alloys, titanium,titanium alloys, cobalt chromium molybdenum alloys, and compositematerials.
 25. The artificial intervertebral disc prosthesis of claim 24wherein said material is an alloy comprising approximately 66 percentcobalt, approximately 28 percent chromium, and approximately 6 percentmolybdenum, by weight.
 26. The artificial intervertebral disc prosthesisof claim 1 wherein said at least one opening of said first endplate andsaid second endplate further comprises a first opening disposed in afirst medial-lateral direction of said first and said second endplatesin a posterior portion thereof, and a second opening in a second,substantially opposite medial-lateral direction of said first and saidsecond endplates in a posterior portion thereof, wherein saidmotion-limiting member is received in said first and second openings.27. The artificial intervertebral disc prosthesis of claim 1 whereinsaid posterior portion of each of said first and second endplatesfurther comprises a concavity that defines posterior lobes projectingfrom said posterior portions of each of said first and second endplates.28. The artificial intervertebral disc prosthesis of claim 1 whereineach of said first and second endplates have an external surfacetherearound defining a generally “D” shape.
 29. The artificialintervertebral disc prosthesis of claim 1 wherein said motion-limitingmember has a length slightly greater than the total distance betweensaid lower surface of said first endplate and said upper surface of saidsecond endplate when said prosthesis is at rest, allowing saidmotion-limiting member to move within said cavity when said prosthesisis subject to loads.
 30. The artificial intervertebral disc prosthesisof claim 1 wherein said motion-limiting member is a cable.
 31. Theartificial intervertebral disc prosthesis of claim 30 wherein said cableis made of a material selected from the group consisting of 316Lstainless steel, MP35N, and Haynes
 25. 32. An artificial intervertebraldisc prosthesis having an anterior portion and a posterior portion,comprising: a first endplate having an upper surface and a lowersurface; a first projection extending from said lower surface of saidfirst endplate terminating in a first distal end; a second endplatehaving an upper surface and a lower surface; a second projectionextending from said upper surface of said second endplate andsubstantially aligned with said first projection, said second projectionterminating at a second distal end to form a gap having a predetermineddistance between said first distal end and said second distal end; and avisco-elastic cushion interposed between said first and second endplatesfurther comprising a cavity for receiving said first and secondprojections.
 33. The artificial intervertebral disc prosthesis of claim32 wherein said first projection of said first endplate extends adistance of approximately 1 to approximately 3 millimeters from saidlower surface of said first endplate.
 34. The artificial intervertebraldisc prosthesis of claim 32 wherein said first projection of said firstendplate is substantially cylindrically shaped.
 35. The artificialintervertebral disc prosthesis of claim 32 wherein said first distal endhas a radius of approximately 2 millimeters to approximately 15millimeters.
 36. The artificial intervertebral disc prosthesis of claim32 wherein said second projection of said second endplate extends adistance of approximately 3 millimeters to approximately 6 millimetersfrom said upper surface of said second endplate to said second distalend.
 37. The artificial intervertebral disc prosthesis of claim 32wherein said gap between said first distal end and said second distalend is approximately 1 millimeters to approximately 2 millimeters. 38.The artificial intervertebral disc prosthesis of claim 32 wherein saidupper surface of said first endplate and said lower surface of saidsecond endplate further comprise appurtenances that aid in securing theprosthesis to adjacent vertebrae.
 39. The artificial intervertebral discprosthesis of claim 32, further comprising a force or pressuretransducer located within said prosthesis for allowing the measurementand transmittal of information about loads experienced by theprosthesis.
 40. The artificial intervertebral disc prosthesis of claim39, wherein said second projection of said second endplate houses atleast a portion of a package of signal conditioning and amplificationelectronics that is connected to said force or pressure transducersplaced within said second projection and at other peripheral locationsaround said second endplate.
 41. The artificial intervertebral discprosthesis of claim 39, wherein said second projection of said secondendplate houses electronics connected to said force or pressuretransducers placed within said second projection and at other peripherallocations around said second endplate.
 42. The artificial intervertebraldisc prosthesis of claim 39, wherein said second endplate furthercomprises a flex circuit including a load sensor embedded onto saidupper surface of said second endplate.
 43. The artificial intervertebraldisc prosthesis of claim 32 wherein said first endplate and said secondendplate comprise a biocompatible material suitable for implantation.44. The artificial intervertebral disc prosthesis of claim 43 whereinsaid first endplate and said second endplate comprise materials selectedfrom the group consisting of stainless steel, stainless steel alloys,titanium, titanium alloys, cobalt chromium molybdenum alloys, andcomposite materials.
 45. The artificial intervertebral disc prosthesisof claim 44 wherein said material is an alloy comprising approximately66 percent cobalt, approximately 28 percent chromium, and approximately6 percent molybdenum, by weight.
 46. The artificial intervertebral discprosthesis of claim 32 wherein said posterior portion of each of saidfirst and second endplates further comprises a concavity that definesposterior lobes projecting from said posterior portions of each of saidfirst and second endplates.
 47. The artificial intervertebral discprosthesis of claim 32 wherein each of said first and second endplateshave an external surface therearound defining a generally “D” shape. 48.An artificial intervertebral disc prosthesis having an anterior portionand a posterior portion, comprising: a first endplate having an uppersurface and a lower surface, said first endplate further comprising atleast one opening; a second endplate having an upper surface and a lowersurface, said second endplate further comprising at least one opening;at least one motion-limiting member received respectively in said atleast one opening of said first and second endplates, said at least onemotion-limiting member connecting said first endplate to said secondendplate and allowing only a predetermined amount of movement of saidfirst endplate relative to said second endplate; and a visco-elasticcushion between said lower surface of said first endplate and said uppersurface of said second endplate, further comprising therein at least onecavity in substantial alignment with said at least one opening in saidfirst and second endplates through which said at least onemotion-limiting member may respectively pass.
 49. The artificialintervertebral disc prosthesis of claim 48 wherein said first endplatefurther comprises a first subplate having an upper surface and a lowersurface and at least one opening therethrough for respectively receivingsaid at least one motion-limiting member, wherein said first projectionof said first endplate extends from said lower surface of said firstsubplate.
 50. The artificial intervertebral disc prosthesis of claim 48wherein said second endplate further comprises a second subplate havingan upper surface and a lower surface, wherein said second projection ofsaid second endplate extends from said upper surface of said secondendplate.
 51. The artificial intervertebral disc prosthesis of claim 48wherein said at least one opening of said first endplate has a firstdiameter near said upper surface of said first endplate and a seconddiameter at said lower surface of said first endplate, wherein saidfirst diameter is greater than said second diameter.
 52. The artificialintervertebral disc prosthesis of claim 48 wherein said at least oneopening of said second endplate has a third diameter at said lowersurface of said second endplate and a fourth diameter at said uppersurface of said second endplate, wherein said third diameter is greaterthan said fourth diameter.
 53. The artificial intervertebral discprosthesis of claim 48 wherein said motion-limiting member is alongitudinal member having a first end and a second end furthercomprising a first enlarged portion at said first end and a secondenlarged portion at said second end.
 54. The artificial intervertebraldisc prosthesis of claim 53 wherein said at least one opening of saidfirst endplate has a first diameter near said upper surface of saidfirst endplate and a second diameter at said lower surface of said firstendplate, and wherein said at least one opening of said second endplatehas a third diameter at said lower surface of said second endplate and afourth diameter at said upper surface of said second endplate, andwherein said first enlarged portion has a diameter greater than saidsecond diameter and wherein said second enlarged portion has a diametergreater than said third diameter.
 55. The artificial intervertebral discprosthesis of claim 48, further comprising a split ring assembly havingan upper surface and a lower surface and a through hole therein.
 56. Theartificial intervertebral disc prosthesis of claim 55 wherein said splitring assembly is fitted into said at least one opening at said uppersurface of said first endplate and at least one opening at said lowersurface of said second endplate.
 57. The artificial intervertebral discprosthesis of claim 56 wherein said through hole at said lower surfaceof said split ring assembly has a smaller diameter than said firstopening of said first endplate at its lower surface.
 58. The artificialintervertebral disc prosthesis of claim 56, wherein said through hole atsaid lower surface of said split ring assembly has a smaller diameterthan said second opening of said second endplate at its upper surface.59. The artificial intervertebral disc prosthesis of claim 56, whereinsaid motion-limiting member comprises a longitudinal member having afirst end and a second end and an enlarged portion at both said firstend and second end, said enlarged portions having a diameter larger thanthat of said opening of said lower surface of said split ring assembly.60. The artificial intervertebral disc prosthesis of claim 48 whereinsaid upper surface of said first endplate and said lower surface of saidsecond endplate further comprise appurtenances that aid in securing theprosthesis to adjacent vertebrae.
 61. The artificial intervertebral discprosthesis of claim 48, further comprising a force or pressuretransducer located within said prosthesis for allowing the measurementand transmittal of information about loads experienced by theprosthesis.
 62. The artificial intervertebral disc prosthesis of claim61, wherein said second projection of said second endplate houses atleast a portion of a package of signal conditioning and amplificationelectronics that is connected to said force or pressure transducersplaced within said second projection and at other peripheral locationsaround said second endplate.
 63. The artificial intervertebral discprosthesis of claim 61, wherein said second projection of said secondendplate houses electronics that are connected to said force or pressuretransducers placed within said second projection or at other peripherallocations around said second endplate.
 64. The artificial intervertebraldisc prosthesis of claim 61, wherein said second endplate furthercomprises a flex circuit including a load sensor embedded onto saidupper surface of said second endplate.
 65. The artificial intervertebraldisc prosthesis of claim 48 wherein said first endplate and said secondendplate are comprised of a biocompatible material suitable forimplantation.
 66. The artificial intervertebral disc prosthesis of claim65 wherein said first endplate and said second endplate are comprised ofmaterials selected from the group consisting of stainless steel,stainless steel alloys, titanium, titanium alloys, cobalt chromemolybdenum alloys, and composite materials.
 67. The artificialintervertebral disc prosthesis of claim 66 wherein said material is analloy comprising approximately 66 percent cobalt, approximately 28percent chrome, and approximately 6 percent molybdenum, by weight. 68.The artificial intervertebral disc prosthesis of claim 48 wherein saidat least one opening of said first endplate and said second endplatefurther comprises a first opening disposed in a left lateral directionin a transverse plane of said first and said second endplates in aposterior portion thereof, and a second opening in a second,substantially opposite right lateral direction in a transverse plane ofsaid first and said second endplates in a posterior portion thereof,wherein a motion-limiting member is received in said first and secondopenings.
 69. The artificial intervertebral disc prosthesis of claim 48wherein said posterior portion of each of said first and secondendplates further comprises a concavity that defines posterior lobesprojecting from said posterior portions of each of said first and secondendplates.
 70. The artificial intervertebral disc prosthesis of claim 69wherein each of said first and second endplates have an external surfacetherearound defining a generally “D” shape.
 71. The artificialintervertebral disc prosthesis of claim 48 wherein said motion-limitingmember has a length slightly greater than the total distance betweensaid lower surface of said first endplate and said upper surface of saidsecond endplate when said prosthesis is at rest, allowing saidmotion-limiting member to move within said cavity when said prosthesisis subject to loads.
 72. The artificial intervertebral disc prosthesisof claim 48 wherein said motion-limiting element is a cable.
 73. Theartificial intervertebral disc prosthesis of claim 72 wherein said cableis made of a material selected from the group consisting of 316Lstainless steel, MP35N, and Haynes
 25. 74. An artificial intervertebraldisc prosthesis having an anterior portion and a posterior portion,comprising: a first endplate having an upper surface and a lowersurface, said first endplate further comprising at least one opening; asecond endplate having an upper surface and a lower surface, said secondendplate further comprising at least one opening; a projection extendingfrom said upper surface of said second endplate terminating at a distalend to form a gap in an unloaded condition between said lower surface ofsaid first endplate and said distal end; at least one motion-limitingmember received respectively in said at least one opening of said firstendplate and said second endplate, said at least one motion-limitingmember linking said first endplate to said second endplate and allowingonly a predetermined amount of movement of said first endplate relativeto said second endplate; and a visco-elastic cushion interposed betweensaid first endplate and said second endplate, further comprising thereinat least one cavity in substantial alignment with said at least oneopening in said first endplate and said second endplate through whichsaid motion-limiting member may pass.
 75. The artificial intervertebraldisc prosthesis of claim 74 wherein said first endplate furthercomprises a first subplate having an upper surface and a lower surfaceand at least one opening therethrough for respectively receiving said atleast one motion-limiting member.
 76. The artificial intervertebral discprosthesis of claim 74 wherein said second endplate further comprises asecond subplate having an upper surface and a lower surface, whereinsaid projection of said second endplate extends from said upper surfaceof said second endplate.
 77. The artificial intervertebral discprosthesis of claim 74 wherein said at least one opening of said firstendplate has a first diameter near said upper surface of said firstendplate and a second diameter at said lower surface of said firstendplate, wherein said first diameter is greater than said seconddiameter.
 78. The artificial intervertebral disc prosthesis of claim 74wherein said at least one opening of said second endplate has a thirddiameter at said lower surface of said second endplate and a fourthdiameter at said upper surface of said second endplate, wherein saidthird diameter is greater than said fourth diameter.
 79. The artificialintervertebral disc prosthesis of claim 74 wherein said projection ofsaid second endplate extends a distance of approximately 3 millimetersto approximately 6 millimeters from said upper surface of said secondendplate to said distal end.
 80. The artificial intervertebral discprosthesis of claim 74 wherein said gap between said distal end and saidlower surface of said first endplate is approximately 1 millimeters toapproximately 2 millimeters.
 81. The artificial intervertebral discprosthesis of claim 74 wherein said motion-limiting member is alongitudinal member having a first end and a second end furthercomprising a first enlarged portion at said first end and a secondenlarged portion at said second end.
 82. The artificial intervertebraldisc prosthesis of claim 81 wherein said first enlarged portion has adiameter greater than said second diameter and wherein said secondenlarged portion has a diameter greater than said third diameter. 83.The artificial intervertebral disc prosthesis of claim 74, furthercomprising a split ring assembly having an upper surface and a lowersurface and a through hole therein.
 84. The artificial intervertebraldisc prosthesis of claim 83 wherein said split ring assembly is fittedinto said at least one opening at said upper surface of said firstendplate and at least one opening at said lower surface of said secondendplate.
 85. The artificial intervertebral disc prosthesis of claim 84wherein said through hole at said lower surface of said split ringassembly has a smaller diameter than said first opening of said firstendplate at its lower surface.
 86. The artificial intervertebral discprosthesis of claim 84, wherein said through hole at said lower surfaceof said split ring assembly has a smaller diameter than said secondopening of said second endplate at its upper surface.
 87. The artificialintervertebral disc prosthesis of claim 84, wherein said motion-limitingmember comprises a longitudinal member having a first end and a secondend and an enlarged portion at both said first end and second end, saidenlarged portions having a diameter larger than that of said aperture ofsaid lower surface of said split ring assembly.
 88. The artificialintervertebral disc prosthesis of claim 74 wherein said upper surface ofsaid first endplate and said lower surface of said second endplatefurther comprise appurtenances that aid in securing the prosthesis tothe adjacent vertebra.
 89. The artificial intervertebral disc prosthesisof claim 74, further comprising a force or pressure transducer locatedwithin said prosthesis for allowing the measurement and transmittal ofload information experienced by the prosthesis.
 90. The artificialintervertebral disc prosthesis of claim 89, wherein said secondprojection of said second endplate houses at least a portion of apackage of signal conditioning and amplification electronics that isconnected to said force or pressure transducers placed within saidsecond projection and at other peripheral locations around said secondendplate.
 91. The artificial intervertebral disc prosthesis of claim 89,wherein said second projection of said second endplate houseselectronics that are connected to said force or pressure transducersplaced within said second projection and at other peripheral locationsaround said second endplate.
 92. The artificial intervertebral discprosthesis of claim 89, wherein said second endplate further comprises aflex circuit including a load sensor embedded onto said upper surface ofsaid second endplate.
 93. The artificial intervertebral disc prosthesisof claim 74 wherein said first endplate and said second endplate arecomprised of a biocompatible material suitable for implantation.
 94. Theartificial intervertebral disc prosthesis of claim 93 wherein said firstendplate and said second endplate are comprised of materials selectedfrom the group consisting of stainless steel, stainless steel alloys,titanium, titanium alloys, cobalt chrome molybdenum alloys, andcomposite materials.
 95. The artificial intervertebral disc prosthesisof claim 94 wherein said material is an alloy comprising approximately66 percent cobalt, approximately 28 percent chrome, and approximately 6percent molybdenum, by weight.
 96. The artificial intervertebral discprosthesis of claim 74 wherein said at least one opening of said firstendplate and said second endplate further comprises a first openingdisposed in a first lateral direction in a transverse plane of saidfirst and said second endplates in a posterior portion thereof, and asecond opening in a second, substantially opposite lateral direction ina transverse plane of said first and said second endplates in aposterior portion thereof, wherein a motion-limiting member is receivedin said first and second openings.
 97. The artificial intervertebraldisc prosthesis of claim 74 wherein said posterior portion of each ofsaid first and second endplates further comprises a concavity thatdefines posterior lobes projecting from said posterior portions of eachof said first and second endplates.
 98. The artificial intervertebraldisc prosthesis of claim 97 wherein each of said first and secondendplates have an external surface therearound defining a generally “D”shape.
 99. The artificial intervertebral disc prosthesis of claim 74wherein said motion-limiting member has a length slightly greater thanthe total distance between said lower surface of said first endplate andsaid upper surface of said second endplate when said prosthesis is atrest, allowing said motion-limiting member to move within said cavitywhen said prosthesis is subject to loads.
 100. The artificialintervertebral disc prosthesis of claim 74 wherein said motion-limitingelement is a cable.
 101. The artificial intervertebral disc prosthesisof claim 100 wherein said cable is made of a material selected from thegroup consisting of 316L stainless steel, MP35N, and Haynes
 25. 102. Anartificial intervertebral disc prosthesis having an anterior portion anda posterior portion, comprising: a first endplate having an uppersurface and a lower surface, said first endplate further comprising atleast one opening for receiving at least one motion-limiting member; afirst projection extending from said lower surface of said firstendplate terminating in a first distal end; a second endplate having anupper surface and a lower surface, said second endplate furthercomprising at least one opening for receiving at least onemotion-limiting member; a second projection extending from said uppersurface of said second endplate and at least partially aligned with saidfirst projection, said second projection terminating at a second distalend to form a gap having a predetermined distance between said firstdistal end and said second distal end; at least one motion-limitingmember received respectively in said at least one opening of said firstendplate and said second endplate, said at least one motion-limitingmember linking said first endplate to said second endplate and allowingonly a predetermined amount of movement of said first endplate relativeto said second endplate; a visco-elastic cushion interposed between saidfirst endplate and said second endplate, further comprising therein atleast one cavity in substantial alignment with said at least one openingin said first endplate and said second endplate through which saidmotion-limiting member may pass; and a force or pressure transducerlocated within the prosthesis for allowing the measurement andtransmittal of the load experienced by the prosthesis.
 103. Theartificial intervertebral disc prosthesis of claim 102 wherein saidsecond projection of said second endplate houses a package of signalconditioning and amplification electronics that is connected to saidforce or pressure transducers placed within said second projection andat other peripheral locations around said second endplate.
 104. Theartificial intervertebral disc prosthesis of claim 102 wherein saidsignal amplification electronics include an internal coil that may beexcited by an inductively coupled external coil.
 105. The artificialintervertebral disc prosthesis of claim 102 wherein said package ofelectronics includes a data storage element for storing load dataexperienced by the prosthesis.
 106. The artificial intervertebral discprosthesis of claim 105, wherein data stored in said data storageelement may be retrieved by a preset sampling routine.
 107. Theartificial intervertebral disc prosthesis of claim 105, wherein datastored in said data storage element may be retrieved remotely andwirelessly via the internet.
 108. The artificial intervertebral discprosthesis of claim 105, wherein said power source for said data storageelement is a micro battery.
 109. The artificial intervertebral discprosthesis of claim 104, wherein said power source for the data storageelement is a capacitor charged from an external inductive couple. 110.The artificial intervertebral disc prosthesis of claim 102 wherein saidforce or pressure transducer is connected to electronics needing nosignal conditioning or amplification circuitry for measuring ortransmitting loads experienced by said prosthesis.
 111. An artificialintervertebral disc prosthesis having an anterior portion and aposterior portion, comprising: a first endplate having an upper surfaceand a lower surface, said first endplate further comprising at least oneopening for receiving at least one motion-limiting member; a firstprojection extending from said lower surface of said first endplateterminating in a first distal end; a second endplate having an uppersurface and a lower surface, said second endplate further comprising atleast one opening for receiving at least one motion-limiting member; asecond projection extending from said upper surface of said secondendplate and substantially aligned with said first projection, saidsecond projection terminating at a second distal end to form a gaphaving a predetermined distance between said first distal end and saidsecond distal end; at least one motion-limiting member receivedrespectively in said at least one opening of said first endplate andsaid second endplate, said at least one motion-limiting member linkingsaid first endplate to said second endplate and allowing only apredetermined amount of movement of said first endplate relative to saidsecond endplate; and a polymeric cushion interposed between said firstendplate and said second endplate, further comprising therein at leastone cavity in substantial alignment with said at least one opening insaid first endplate and said second endplate through which saidmotion-limiting member may pass and at least one cavity surrounding saidfirst and second projections.
 112. An artificial intervertebral discprosthesis having an anterior portion and a posterior portion,comprising: a first endplate having an upper surface and a lowersurface; a first projection extending from said lower surface of saidfirst endplate terminating in a first distal end; a second endplatehaving an upper surface and a lower surface; a second projectionextending from said upper surface of said second endplate andsubstantially aligned with said first projection, said second projectionterminating at a second distal end to form a gap having a predetermineddistance between said first distal end and said second distal end; and apolymeric cushion interposed between said upper and lower endplatesfurther comprising a cavity for receiving said first and secondprojections.
 113. An artificial intervertebral disc prosthesis having ananterior portion and a posterior portion, comprising: a first endplatehaving an upper surface and a lower surface, said first endplate furthercomprising at least one opening for receiving at least onemotion-limiting member; a second endplate having an upper surface and alower surface, said second endplate further comprising at least oneopening for receiving at least one motion-limiting member; at least onemotion-limiting member received respectively in said at least oneopening of said first and second endplates, said at least onemotion-limiting member connecting said first endplate to said secondendplate and allowing only a predetermined amount of movement of saidfirst endplate relative to said second endplate; and a visco-elasticcushion between said lower surface of said upper endplate and said uppersurface of said lower endplate, further comprising therein at least onecavity in substantial alignment with said at least one opening in saidfirst and second endplates through which said at least onemotion-limiting member may respectively pass.
 114. An artificialintervertebral disc prosthesis having an anterior portion and aposterior portion, comprising: a first endplate having an upper surfaceand a lower surface, said first endplate further comprising at least oneopening for receiving at least one motion-limiting member; a secondendplate having an upper surface and a lower surface, said secondendplate further comprising at least one opening for receiving at leastone motion-limiting member; a projection extending from said uppersurface of said second endplate and substantially aligned with saidfirst projection, said projection terminating at a distal end to form agap having a predetermined distance between said lower surface of saidfirst endplate and said distal end; at least one motion-limiting memberreceived respectively in said at least one opening of said firstendplate and said second endplate, said at least one motion-limitingmember linking said first endplate to said second endplate and allowingonly a predetermined amount of movement of said first endplate relativeto said second endplate; and a polymeric cushion interposed between saidfirst endplate and said second endplate, further comprising therein atleast one cavity in substantial alignment with said at least one openingin said first endplate and said second endplate through which saidmotion-limiting member may pass.